Field of the Invention
The present invention relates to a numerical controller, and more particularly, to a numerical controller with the function of performing corner control of minute blocks.
Description of the Related Art
During machining by a wire electrical discharge machine, a wire electrode 2 is deflected (wire electrode 2′) in the direction opposite to a machining progress direction 3 by a discharge repulsive force or a machining fluid flow, as shown in FIGS. 9A and 9B. In FIG. 9B, reference numerals 3a and 3b denote upper and lower wire guides, respectively, and reference numeral 4 denotes a workpiece.
If machining is performed without the consideration of the influence of the deflection of the wire electrode 2, inward turning occurs at corner portions and circular-arc portions due to the deflection (wire electrode 2′) of the central portion of the wire electrode 2 with respect to the thickness direction of the workpiece (vertical direction in FIG. 9B). Thus, a command shape involves bites and residues, as shown in FIG. 10A (corner portion) and FIG. 11A (circular-arc portion). In FIG. 10A and its subsequent drawings, reference numeral 2a denotes a command position of the wire electrode 2, and reference numeral 2b denotes the position of the central portion of the wire electrode 2 shifted by the deflection, with respect to the thickness direction of the workpiece.
Japanese Patent Applications Laid-Open Nos. 58-120428, 05-228736, 2014-148036, etc., disclose conventional techniques to overcome the problem that the command shape involves bites and residues due to the deflection of the wire electrode. According to these techniques, automatic adjustment of machining conditions (electrical discharge conditions) and a machining speed (machining condition control) and automatic correction of a machining path (machining path correction) are performed at corner portions and circular-arc portions.
By these conventional techniques, the machining conditions are changed when the corner portion and the circular-arc portion of the machining path are approached by the wire electrode 2, as shown in FIG. 10B (corner portion) and FIG. 11B (circular-arc portion), and the amount of deflection is reduced by controlling the discharge voltage and the pressure or amount of a machining fluid to reduce a pressure on the wire electrode 2. In this way, the bites and residues at the corner portion and the circular-arc portion can be reduced.
Also, the bites and residues at the corner portion and the circular-arc portion can be reduced by correcting a command path for the wire electrode 2 so that a deflected portion of the wire electrode can move along the command path, as shown in FIG. 10C (corner portion) and FIG. 11C (circular-arc portion).
The machining condition control and the machining path correction described above are performed based on shape data (block length, corner angle, arc radius, arc central angle, etc.) on the machining path calculated by a numerical controller.
In machining a free curve shape, a command shape is generally approximated by minute blocks, such as minute line segments or minute arcs. Such a command shape is generated based on an analysis of the free curve shape by a CAD/CAM system or the like.
The conventional techniques in which the machining condition control and the machining path correction are performed at the corner portions and the circular-arc portions are configured so that corner shape data is calculated block by block. Therefore, the following problems occur if the minute line segments or minute arcs used in the machining of the free curve shape are interpolated.
[Problem 1] Arc control cannot be performed even for a machining path having a curve shape with a small curvature radius:
In approximating a free curve shape by minute line segments or minute arcs, the curve shape is approximated by a shape including a plurality of minute line segments. There is a problem that if a set of such minute line segments (part (a) shown in FIG. 12A) is directly interpolated, an interpolation path is polygonal-line-shaped, as shown in FIG. 12B, so that arc control cannot be achieved.
[Problem 2] A necessary control distance for corner control cannot be secured:
A certain control distance is needed to achieve corner control at a corner portion (part (b) shown in FIG. 12A) by the conventional techniques. Since the corner shape data is calculated block by block according to the conventional techniques, however, there is a problem that the necessary control distance for corner control cannot be secured at an acute corner between the minute blocks, as shown in FIG. 12C.
[Problem 3] Calculation errors and the like of the CAD/CAM system cannot be overcome:
If the free curve shape is analyzed by the CAD/CAM system and approximated by minute line segments or minute arcs, a very minute difference in level (part (c) shown in FIG. 12A) may sometimes be created in the command shape due to calculation errors or the like. There is a problem that if the conventional corner control is directly applied to the command shape, an acute corner is inevitably subjected to the corner control for a substantially smooth curve shape, as shown in FIG. 12D.
There is an additional problem that if a method is employed in which curves (spline curve, NURBS curve, quadratic curve other than a circular arc, etc.) obtained by smoothing minute line segments or minute arcs are interpolated, instead of directly interpolating the minute line segments or minute arcs, no conventional techniques can perform the machining condition control and the machining path correction based on shape data on these curves, so that proper corner control and arc control cannot be achieved.